x86, mce: fix build failure in arch/x86/kernel/cpu/mcheck/threshold.c

Impact: build fix

The APIC code rewrite in the x86 tree broke the x86/mce branch:

 arch/x86/kernel/cpu/mcheck/threshold.c: In function ‘mce_threshold_interrupt’:
 arch/x86/kernel/cpu/mcheck/threshold.c:24: error: implicit declaration of function ‘ack_APIC_irq’

Also tidy up the file a bit while at it.

Signed-off-by: Ingo Molnar <mingo@elte.hu>